Shelf Sea Biogeochemistry blog

Thursday, 27 November 2014

Heading north

Ocean research cruise blog of Jonathan Sharples

Another successful day yesterday, with the wirewalker mooring and
both of the gliders recovered very quickly. Jo Hopkins immediately
removed all of the instruments from the wirewalker, and strapped them to
the CTD ready for the next time we lowered it through the water. This
allows Jo to calibrate the wirewalker data with the data collected by
the CTD, with the CTD data all calibrated against analysis of samples we
collect in the sample bottles. Every profile of data we collect through
the water with the CTD involves samples being collected for salt
concentration, dissolved oxygen and chlorophyll. These samples are
analysed against known, internationally-recognised standards and lab
techniques, so that we can calibrate the sensors on the CTD and estimate
the error associated with their measurements. This is a vital part of
any science: no other scientist would allow us to publish our results if
we couldn’t demonstrate that our measurements achieved acceptable
standards.

omg glider recovery

We can measure salt concentration to within about 2 thousandths of a
gramme in 1 kg of seawater. We need to know salt to this level of
accuracy because it has, along with temperature, a big influence on how
dense the seawater is. The sea is always attempting to sort itself out
so that less dense water floats above denser water, so knowing salt and
temperature can tell us a lot about how the water will be moving. I’ve
mentioned dissolved oxygen before in the context of Chata’s work –
biology both produces oxygen (when the microbial plants are glowing) and
consumes oxygen (when bacteria break down the organic matter), so
accurate data on the oxygen in the water tells us a lot about how the
biology is operating. Chlorophyll in the ocean is the same green stuff
that you see in leaves and grass – the chemical that plants use to
collect energy from sunlight. Chlorophyll is particularly good for
plants that live in the ocean. Sunlight is absorbed very quickly as it
passes downward from the sea surface. All of the red light from the sun
is absorbed within the first 1 metre below the sea surface. Blue light
travels the deepest in the sea, and chlorophyll is well suited to
capturing energy from blue light. Clearly this is an advantage for the
microbial plants in the sea, as they are mixed through the upper few 10s
of metres and need to maximise their chances of collecting the sun’s
energy. But why should land-based plants use chlorophyll when they don’t
have the problem of metres of ocean absorbing the light? Photosynthesis
first evolved in the ocean. Land-based plants haven’t bothered to
evolve a form of photosynthesis more suited to life above the sea,
instead they just highjacked the system that the ocean’s microbial
plants had developed. Quite literally. At the heart of the
photosynthesising biochemical machinery in every leaf lies a
light-capturing system that can be genetically traced right back to
photosynthesising marine bacteria.

Billy does the salts

We’ve started to head north through the Celtic Sea now, stopping
every 25 km or so to lower the CTD through the water and collect more
information. The wind has picked up, with about 25-30 knots now. The sea
is looking rough, but it’ll take a few hours for the swell to pick up
and start to move us about.